Modern motor vehicles have driver assistance systems which assist the driver in various driving situations by actively intervening in the control of the vehicle in situations in which the stability of the vehicle is at risk. The driver assistance systems include inter alia devices for controlling the vehicle movement dynamics, which are also referred to as electronic stability control (ESC) or an electronic stability program (ESP). In this context, ESP extends and links the known anti-lock brake system (ABS) with traction control (TC) and electronic braking force distribution. The traction control ensures that one or more wheels of a motor vehicle do not spin during acceleration and when the adhesive friction is at the same time low, e.g., in the case of a slippery underlying surface, wet underlying surface etc.
A characteristic variable that constitutes an important characteristic variable for vehicle movement dynamics control is what is referred to as the wheel slip. The wheel slip denotes the ratio between the distance actually traveled per rotation of the wheel and the actual circumference of the wheel. In the case of a low slip, the wheels grip the coating of the underlying surface very well and can therefore move the vehicle. Conversely, a high slip characterizes situations in which the wheels no longer have complete contact with the underlying surface. The locking of the wheels as a result of a high braking slip and the spinning of the wheels is therefore characterized by high drive slip. It is necessary to avoid excessive braking slip and drive slip, since they can bring about undesired instability of the motor vehicle.
Known anti-lock brake systems compare a measured wheel speed with an estimated vehicle reference speed and calculate therefrom the absolute wheel slip S_abs or the relative wheel slip S_rel, as shown in equations 1 and 2:S_abs=V_Veh+V_Wh  (1)S_rel=(V_Veh−V_Wh)/V_Veh  (2)
In addition to the conventional braking slip, in which the wheels tend to lock as a result of excessively strong braking, calculated slip can also occur when the motor vehicle is moving along a curved path. In this case, the individual wheels rotate at different speeds solely as a result of their different curve radiuses. Therefore, wheel-specific slip occurs which is induced by a curve radius and which is dependent on the position of the wheel on the motor vehicle. This slip is referred to, at least in this patent application, as geometric slip. The geometric slip is based inter alia on the following effects:
1. The wheels of the motor vehicle on the outside of the bend move on a larger curve radius than the wheels on the inside of the bend and therefore travel a larger distance in the same time. This brings about a higher wheel speed of the wheels on the outside of the bend.
2. The wheels of the non-steered rear axle move along a tighter curve radius and therefore more slowly than the wheels on the steered front axle. This effect is all the more serious in the case of very small curve radiuses and/or at relatively low vehicle velocities.
3. Owing to centrifugal forces, higher wheel loads act on the wheels on the outside of the bend than on the wheels on the inside of the bend, as a result of which the dynamic wheel radius becomes smaller. As a result of this at a specific velocity the wheels rotate at an increased rotational speed. However, since the ABS device does not determine the actual wheel speed on the basis of the distance traveled but instead determines the wheel speed only indirectly by means of the rotational speed of the wheel, the wheel speeds which are determined by the ABS device are higher at the wheels on the outside of the bend than at the wheels on the inside of the bend.
In specific driving situations, in particular when the velocity is very low and there are tight bends, this difference in speed between the wheels on the outside of the bend and the wheels on the inside of the bend can assume orders of magnitude of up to 6 km/h. This effect should therefore not be neglected, in particular when configuring the slip threshold at low speeds. Otherwise, when traveling around a bend at a low velocity the wheels on the inside of the bend would rotate more slowly, solely owing to the travel around the bend, to such an extent that the ABS control is actuated without an ABS-relevant event actually occurring. In order to avoid this situation, there is now the possibility of configuring the slip threshold with a correspondingly high value. However, this more robust configuration of the slip thresholds contradicts the need for ABS control to be as sensitive as possible.
In this respect, there is the need to differentiate the situation of “straight-ahead travel” from “travel around a bend” (geometric slip) when configuring the slip thresholds. By using a suitable additional sensor system, for example a yaw rate sensor, lateral acceleration sensor, steering sensor, etc., given knowledge of the vehicle geometry it will be possible to calculate very precisely the difference in speed caused by the travel around the bend. It is problematic if such an additional sensor system is not available or is, for example, defective.